A typical rocket motor has a pressure vessel (motor case) and a nozzle. The motor case contains the propellant. In typical rocket motor configurations, the nozzle and nozzle components are housed in an aft closure. In addition to holding the nozzle and nozzle components, the aft closure also seals an end of the motor case so that the seal is able to withstand the pressure generated inside the rocket motor case. To meet this goal, the aft closure is made from a single piece of material. FIG. 1 illustrates a prior art aft closure 100 with blast tube 102. The aft closure 100 is made from a single (i.e., a monolithic) piece of material. The aft closure 100 is connected to the blast tube 102 either by being milled from the same piece of material as the aft closure, or alternatively, the blast tube 102 is E-beam welded to the aft closure 100. The cylindrical wall 104 exterior seals against the inside surface of the motor case. To this end, the cylindrical wall 104 includes a groove 106 for an O-ring (not shown).
Because tactical rocket motors are designed to perform over a broad range of temperatures (e.g., −65° F. to 160° F.), the motor case and aft closure 100 are typically made from materials having similar coefficients of thermal expansion (CTE). Using the same materials or materials with similar coefficients of thermal expansion are necessary to reduce leaks between the motor case and the aft closure 100.
Ideally, the motor case and aft closure are manufactured from the same material. Titanium is a common material used for the motor case because of its relatively high strength to weight ratio. Minimizing the amount of material is also a design consideration, making the use of high-strength, low-weight materials, such as titanium, an attractive option for the closure 100 as well. Manufacturing the aft closure 100 from titanium reduces the mass of the closure, but also has inherent disadvantages. For example, the cost of the raw material is much higher for titanium than steel or aluminum. Also, an aft closure 100 has intricate details that are difficult to produce in a single piece of titanium, leading to expensive manufacturing processes.
On the other hand, avoiding a titanium aft closure 100 and manufacturing the aft closure 100 from a light, inexpensive, easily machined material, such as aluminum, has other disadvantages. For example, aluminum has a high coefficient of thermal expansion as compared to materials used for the motor case. Consequently, extremes in temperature will create potential leak conditions. Further, aluminum has a lower strength than steel or titanium, requiring more mass to achieve similar strength as titanium.